It is known that the important optical-fiber transmissive features of principal interest for telecommunications are attenuation and dispersion, which depend upon wavelength.
Attenuation determines repeater spacing and dispersing the amount of information which can be transmitted in a unit of time.
Total attenuation in a fiber depends on various factors, some of which depend in turn on the material characteristics. Among those factors are:
Rayleigh scattering, causing an attenuation given by the relation a.sub.sc =a.sub.o k.sup.-4 where k=wavelength and a.sub.o =scattering coefficient; and
lattice bond absorption, causing an attenuation given by the relation a.sub.abs =A exp (-ck.sub.1 /k) where A, c are constants, and k.sub.1 is a constant of the medium (wavelength of fundamental stretch vibration of lattice bond).
These two phenomena together generate a total attenuation whose representative curve is V-shaped, with a vertex substantially coinciding with the minimum attenuation point.
In the case of silica-glass monomode fibers, which are the most widely used, the minimum attenuation point is close to 1.55 .mu.m wavelength; the minimum dispersion point (at which the dispersion is basically zero) is, by contrast, about 1.27 .mu.m, at which attenuation is about twice the minimum.
The choice of operating at either wavelength depends on the characteristics of the available sources, and corresponds to giving most importance to either repeater spacing or transmission rate, respectively. Hence the importance is clear of producing fibers in which the two minima are very closely spaced or even coincident, so as to optimize both characteristics.
The minimum dispersion point can be varied by acting on fiber material refractive-index, and silica-glass fiber production methods have been suggested in which the minimum dispersion point is shifted towards higher wavelengths and is practically made to coincide with that of minimum attenuation.
An example is described in the paper entitled: "Low-loss dispersion-shifted single mode fiber manufactured by the OVD process" presented by T. D. Croft, J. E. Ritter and V. A. Bhagavatula at the Conference on Optical Fiber Communication, San Diego (California, U.S.A.), paper WD2, a digest of which is published at page 94 of the conference proceedings.
The shift of minimum dispersion is obtained by successively depositing a GeO.sub.2 -doped silica inner core with a triangular refractive-index profile, a region of pure silica, a GeO.sub.2 -doped silica outer core with stepped refractive-index profile, and finally a pure silica cladding.
The known method is unsatisfactory because of its complexity, since it requires sophisticated refractive-index variations, and because of the fact that dopant addition increases the overall attenuation by comparison with a conventional fiber.